Elucidating the impacts of cobalt (II) ions on extracellular electron transfer and pollutant degradation by anodic biofilms in bioelectrochemical systems during industrial wastewater treatment.

Electrogenic biofilms in bioelectrochemical systems (BES) are critical in wastewater treatment. Industrial effluents often contain cobalt (Co2+); however, its impact on biofilms is unknown. This study investigated how increasing Co2+ concentrations (0-30 mg/L) affect BES biofilm community dynamics, extracellular polymeric substances, microbial metabolism, electron transfer gene expression, and electrochemical performance. The research revealed that as Co2+ concentrations increased, power generation progressively declined, from 345.43 ± 4.07 mW/m2 at 0 mg/L to 160.51 ± 0.86 mW/m2 at 30 mg/L Co2+. However, 5 mg/L Co2+ had less effect. The Co2+ removal efficiency in the reactors fed with 5 and 10 mg/L concentrations exceeded 99% and 94%, respectively. However, at 20 and 30 mg/L, the removal efficiency decreased substantially, likely because of reduced biofilm viability. FTIR indicated the participation of biofilm functional groups in Co2+ uptake. XPS revealed Co2+ presence in biofilms as CoO and Co(OH)2, indicating precipitation also aided removal. Cyclic voltammetry and electrochemical impedance spectroscopy tests revealed that 5 mg/L Co2+ had little impact on the electrocatalytic activity, while higher concentrations impaired it. Furthermore, at a concentration of 5 mg/L Co2+, there was an increase in the proportion of the genus Anaeromusa-Anaeroarcus, while the genus Geobacter declined at all tested Co2+ concentrations. Additionally, higher concentrations of Co2+ suppressed the expression of extracellular electron transfer genes but increased the expression of Co2+-resistance genes. Overall, this study establishes how Co2+ impacts electrogenic biofilm composition, function, and treatment efficacy, laying the groundwork for the optimized application of BES in remediating Co2+-contaminated wastewater.

Heterotrophic anodic denitrification coupled with cathodic metals recovery from on-site smelting wastewater with a bioelectrochemical system inoculated with mixed Castellaniella species.

Although Castellaniella species are crucial for denitrification, there is no report on their capacity to carry out denitrification and anode respiration simultaneously in a bioelectrochemical system (BES). Herein, the ability of a mixed inoculum of electricigenic Castellaniella species to perform simultaneous denitrification and anode respiration coupled with cathodic metals recovery was investigated in a BES. Results showed that 500 mg/L NO3--N significantly decreased power generation, whereas 100 and 250 mg/L NO3--N had a lesser impact. The single-chamber MFCs (SCMFCs) fed with 100 and 250 mg/L NO3--N concentrations achieved a removal efficiency higher than 90% in all cycles. In contrast, the removal efficiency in the SCMFCs declined dramatically at 500 mg/L NO3--N, which might be attributable to decreased microbial viability as revealed by SEM and CLSM. EPS protein content and enzymatic activities of the biofilms decreased significantly at this concentration. Cyclic voltammetry results revealed that the 500 mg/L NO3--N concentration decreased the redox activities of anodic biofilms, while electrochemical impedance spectroscopy showed that the internal resistance of the SCMFCs at this concentration increased significantly. In addition, BES inoculated with the Castellaniella species was able to simultaneously perform heterotrophic anodic denitrification and cathodic metals recovery from real wastewater. The BES attained Cu2+, Hg2+, Pb2+, and Zn2+ removal efficiencies of 99.86 ± 0.10%, 99.98 ± 0.014%, 99.98 ± 0.01%, and 99.17 ± 0.30%, respectively, from the real wastewater. Cu2+ was bio-electrochemically reduced to Cu0 and Cu2O, whereas Hg0 and HgO constituted the Hg species recovered via bioelectrochemical reduction and chemical deposition, respectively. Furthermore, Pb2+ and Zn2+ were bio-electrochemically reduced to Pb0 and Zn0, respectively. Over 89% of NO3--N was removed from the BES anolyte during the recovery of the metals. This research reveals promising denitrifying exoelectrogens for enhanced power generation, NO3--N removal, and heavy metals recovery in BES.